1. Field
The present application relates generally to the operation and design of transceivers, and more particularly, to intermodulation (IM) distortion reduction.
2. Background
High quality signal reception is especially important for the current generation of portable devices. Typically, such devices provide multiple services, such as wireless communication services and, for example, position location services that requires the reception of global positioning system (GPS) signals. Thus, the front end of a device's receiver needs to be carefully designed to reject interfering signals, referred to as jammers or jamming signals, and receive desired signals with high quality.
To illustrate the problem, consider a GPS coexistence scenario where a portable device includes a GPS receiver and a cellular transmitter. In this GPS coexistence scenario, strong radio frequency second order intermodulation (IM2) distortions can be generated from signals at the front end of the GPS receiver due to transmissions on the cellular channel. For example, the GPS L1 band is at approximately 1575.42 MHz and a 2nd order intermodulation harmonic associated with B13 (Uplink: 777-787 MHz) of an LTE (Long Term Evolution) communication system will fall into this GPS L1 band. It would therefore be desirable to address such IM2 distortions to enhance GPS system performance.
Many of the existing solutions attempt to reduce IM2 distortions generated during and/or after frequency down-conversion (post-mixer). These distortions can be referred to as baseband IM2 (BB-IM2). Unfortunately, in numerous scenarios IM2 distortion is generated prior to the down-conversion stages (pre-mixer), such as in WiFi 802.11b/g/n and GSM850 Band Coexistence (2400 MHz-825 MHz), or the 2nd harmonic of B13 (Uplink: 777-787 MHz) LTE. Thus, for example, the common mode portion of a jamming signal may intermodulate with its own differential portion, which may generate systematic pre-mixer IM2 at the differential input of an LNA in the receiver's front end. Such IM2 distortion may not be canceled out by conventional calibration schemes designed to reduce BB-IM2. Other techniques utilized to reduce pre-mixer IM2 include additional filtering at the front end which may help, but also increases the number of required circuit components and can add additional noise to the system.
Therefore, it would be desirable to have an efficient and cost effective way to reduce or eliminate systematic pre-mixer IM2 distortion in a receiver front end.